Major Number & Minor Numbers used by hardware devices under both Linux and UNIX. Generally stored in /dev/ directory these are files and not actual device driver (they are in kernel itself or as module)

Basically Major Number & Minor Numbers are pointers for devices which give idea about that device.

Major number is use to call driver and Minor number represents which device you want to use. For example IDE driver can be use to access two IDE hard disk connected at different IDE0 and IDE1 so minor number use to determine which drive you would like to use.

For example
IDE Major number 3 and minor number 0 represent 1st IDE hard disk
IDE Major number 3 and minor number 1 represent 2ndt IDE hard disk

Remember a device node consists of a name, a type (block, character, etc), and a major and minor number.